A ceramic substrate has hitherto widely been used for a wiring board for a multi-chip module package in which a plurality of semiconductor elements and a plurality of passive elements such as a capacitor and a resistor are mounted. A low temperature co-fired glass ceramic substrate (LTCC substrate), which is obtained by firing at low temperature, or the like is used as the ceramic substrate. The “low temperature” in the case of the low temperature co-fired glass ceramic substrate refers to a temperature range of from 850° C. to 1,000° C. The LTCC substrate includes an insulating base material formed of glass ceramic, and a wiring pattern formed of a sintered body containing as a main component a metal material (for example, silver) (wiring pattern formed of a silver sintered body).
The wiring pattern formed of a silver sintered body in the LTCC substrate is electrically connected to a semiconductor element or a passive element by wire bonding, and then connected to a printed board made of a resin serving as an external electrical circuit via solder. In general, a multi-layered electroless plating coating that satisfies both a wire bonding property and solderability necessary for the connection is formed on a surface of the wiring pattern. As constituents of the multi-layered electroless plating coating, a nickel plating coating, a gold plating coating, and in recent years, a palladium plating coating have been known.
As a method of forming the nickel plating coating, the gold plating coating, and the palladium plating coating selectively on the wiring pattern formed of a silver sintered body in the LTCC substrate, electroless plating has been widely used. The electroless plating generally involves a plurality of steps including a surface activation step, a catalyzing step, and a plurality of electroless plating steps of forming a multi-layered plating coating including an electroless nickel plating coating and the like.
In association with formation of the plating coating on the surface of the wiring pattern formed of a silver sintered body in the LTCC substrate, it has hitherto been known that, at the time of firing of the substrate, a phenomenon called glass floating, in which a glass component is formed on the surface of the wiring pattern formed of a silver sintered body, may occur. When the glass floating occurs, a non-plating state, in which the nickel plating coating is prevented from being formed on the glass component, may occur, or the plating coating may have an uneven thickness.
As a method of preventing such non-plating state, there has hitherto been known, for example, a method involving, between the surface activation step and the catalyzing step, a step of removing the glass component present on a surface to be subjected to electroless plating by using a pretreatment agent containing a reducing agent (see, for example, Patent Literature 1).